The present invention relates to an electrical inductive load drive control which is applicable to, for instance, an electronic throttle control apparatus for vehicles for driving a throttle valve disposed in an engine intake pipe by a direct current motor in accordance with an accelerator pedal position and the like.
An H-bridge drive circuit is conventionally used as a load drive apparatus for driving an electrical load such as a direct current motor having inductive coils. In this load drive device, four switching devices are diagonally connected to cross each other. The energization or deenergization of the load are switched by turning on or off the switching devices at certain duty ratios.
It is necessary to extinguish energy which is generated in the coil of the direct current motor, when the switching device is turned off to change the direct current motor from the energized condition to the deenergized condition.
In JP-A-9-18313, it is proposed to turn on two switching devices at high side or low side to form a closed loop including the load, thereby extinguishing the energy generated in the coil of the direct current motor through the closed loop. It is also proposed to connect diodes in parallel with the drive terminals of the switching device to extinguish the energy generated in the coil. For instance, the body diode integrally built in a MOSFET is forward-biased when the direct current motor is deenergized, thereby extinguishing the generated energy between a power source and the ground through the body diode.
MOSFETs are generally used as switching devices. In the MOSFET, as shown in FIG. 12A, power loss Pm between its drain and its source increases remarkably as electric current between its drain and its source. The power loss Pm of the MOSFET at the time of energy extinction increases in proportion to the square of the current I and the on-resistance R of the MOSFET, that is, Pm=R.times.I.sup.2.
In diodes, a forward voltage Vf increases gradually as a forward current I increases, so that power loss Pd in the forward direction increases as shown in FIG. 12B, that is, Pd=Vf.times.I.
Accordingly, the power loss Pm increases remarkably as the current I increases at the time of deenergization of the direct current motor in the case of extinguishing the energy through the closed loop, while the power loss Pm is rather small as long as the current I is relatively low. The change rate of power loss Pd is relatively low at the time of deenergization of the direct current motor in the case of the diode when the current I is low, while the power loss Pd is relatively low in comparison with the case of the closed loop when the current I is high.
If the low power loss should be reduced over a wide range of current I, the MOSFET must be sized large in chip size for a low on-resistance over the wide current range in the case of the closed loop. The MOSFET must be sized large to have a large allowable power loss in the case of the diode. In either case, the switching devices become expensive.